1. Field of the Invention
This invention relates to any method wherein salts dissolved in a water supply are concentrated in a residual portion thereof, and, in particular, to a method for the pretreatment of the water supplied to a desalinization process and the brine produced therein in a fashion which greatly increases the operational efficiency of the desalinization method.
2. Brief Statement of the Prior Art
Expanding urban populations and the ever increasing reclamation of arid lands by irrigation have resulted in the maximum utilization of available waters in the arid and semi-arid areas of the country. Additionally, recognition of the right of subsequent users of surface waters, streams and the like to waters of undiminished quality has resulted in the necessity for the purification of various saline waters. Among such applications are the treatment of saline water recovered in the drains of irrigated lands to recondition this drain water for return to the water-shed, and the processing of industrial waste waters to reduce their impurities.
Various methods have been proposed for desalinization of saline water such as agricultural drain waters or industrial waste waters. Such treatments include distillation, reverse osmosis, electrodialysis, freezing, ion exchange and vapor compression which all effect separation of a purified water of greatly reduced salt content from a brine that is concentrated in the impurities present in the saline water. When the saline water is purified, the impurities in the brine rapidly reach their solubility limits at the treatment conditions and the brine becomes super-saturated. The super-saturated brine presents a number of difficulties in the purification treatment; it can cause the formation of scale on equipment such as the heating surfaces used in distillation, the heat exchange surfaces used for cooling of the water in freezing purifications, or the equipment surfaces used in vapor compression. Additionally, the efficiency of membrane processes such as reverse osmosis or electrodialysis is greatly reduced by scaling the membrane and precipitation formation in the brine channels of the equipment. In ion exchange water treatments, sulfuric acid is commonly used to regenerate spent cationic resins and the presence of calcium in the spent resin can result in the precipitation of calcium sulfate in the resin during its regeneration.
The practice followed to avoid operational difficulties caused by the limited solubility of the impurities, typically by the limiting solubility of calcium sulfate, is to limit the concentration of the brine in the purification process, thereby necessitating the treatment of substantially greater quantities of the saline water than is desirable.
In reverse osmosis units, agricultural drain water can only be concentrated about three-fold before the precipitates of calcium sulfate greatly scale the membrane and plug the equipment brine channels in the reverse osmosis unit. As a result, most reverse osmosis processes on saline waters require the discharging of approximately 25 percent of the water processed to waste facilities, thereby inhibiting the efficiency of the process and presenting a disposal problem of considerable magnitude.
Various techniques have been suggested for the pretreatment of saline water to improve the efficiency of their subsequent desalinization treatment. One method, described in U.S. Pat. No. 3,262,865, discloses that the deposition of scale in a water distillation process can be inhibited by acidifying the raw water with sulfuric acid. Another pretreatment method, disclosed in U.S. Pat. No. 3,639,231, comprises the ion exchange of the raw water prior to its desalinization treatment with reverse osmosis. A number of processes have also been developed for a chemical water softening of waters which contain hardness ions such as calcium and magnesium. Typically, these processes comprise the addition of calcium hydroxide alone or in combination with sodium carbonate to reduce the solubility of the dissolved calcium carbonate in the water. Typical of such treatments is that disclosed in U.S. pat. No. Pat. 3,740,330. These treatments, however, are not generally applicable to desalinization treatments because they do not sufficiently reduce the concentration of scale-forming dissolved salts in the processed water.